1. Field of the Invention
The present invention relates to a pacer system which is adapted to alter the rate of pacer pulses delivered (rate of pacing pulses delivered by an artificial pacemaker) to a heart while an individual is exercising relative to, and utilizing, the partial pressure of oxygen, pO.sub.2, in the blood in a heart to obtain a required cardiac output.
2. Descriptiion of the Prior Art
Heretofore patients with heart dysfunctions or heart disease such as sinus node disease have been able to live a relatively normal life with the implantation of an artificial pacemaker often referred to as a pacer. However, such pacers have not always been able to mimic the response of a normal healthy heart. A normal heart responds to exercise and stress by increasing cardiac output through increased heart rate and/or stroke volume.
In this respect, patients with sinus node disease have lost the ability to increase heart rate with exercise. Accordingly, it has become a goal of optimal pacing to provide exercise responsiveness in a pacer by sensing the need for increased cardiac output.
With a view towards obtaining this goal, a number of pacemakers have been proposed for indirectly sensing the need for increased heart rate by sensing P-waves, nerve impulses, Q-T interval, pH, oxygen saturation, respiratory rate, stroke volume, motion, atrial pressure and temperture.
A P-wave triggered artificial pacemaker adapted to be exercise responsive by responding to average atrial rate is proposed in the Knudson & Amundson U.S. Pat. No. 4,313,442.
An artificial pacemaker responsive to changes in the Q-T interval is proposed in the Rickards U.S. Pat. No. 4,228,803.
The Funke U.S. Pat. No. 4,312,355 discloses a dual pace-dual sense cardiac pacer which is able to stimulate the atrium and/or the ventricle and which is able to entrain the ventricle, when the atrial rate increases, while preventing bradycardic episodes. The pacer action is triggered by detection of naturally occurring atrial and ventricular action or pulses within a predetermined time period.
The Roline U.S. Pat. No. 4,363,325 discloses a multiple mode pacer activated to switchmodes relative to heart rate while preventing atrial bradycardia. This is achieved by mode switching of the pacer from an atrial synchronous mode to a vetricular inhibited mode. Such switch of modes is actuated when no atrial activity is sensed within a preset escape interval referred to as a hysteresis period. Reversal of the mode back to the atrial synchronous mode from the ventricular inhibited mode is actuated in response to a detected atrial rate which is higher than a preset, lower, ventricualr rate. With this mode switching, the ventricle will not be stimulated twice in quick succession, which overstimulation could cause atrial bradycardia.
The Heilman et al. U.S. Pat. No. 4,303,075 discloses a method and apparatus for maximizing stroke volume through AV pacing using an implanted cardioverter/pacer which is programmed with an AV delay tailored to the particular patient. The mechanism detects and processes the impedance of the heart across two electrodes in contact with heart muscle during each heart cycle and uses the changes from cycle to cycle to trigger the issuance of pulses from the pacer depending on the direction of the impedance changes to maximize stroke volume of the heart, which is proportional to the change in value of impedance between the minimum and maximum detected impedance per heart cycle.
A proposal for placing electrodes on the Hering's nerve which extends from receptors in the sinus and glomus carotids is disclosed in the Gonzalez U.S. Pat. No. 4,201,219.
Sensors for sensing blood pH are proposed in the Alcidi U.S. Pat. No. 4,009,721 and the Mauer et al U.S. Pat. No. 4,252,124. Alcidi controls a pacer relative to the level of blood pH sensed.
See also the article "A new pacemaker autoregulating the rate of pacing in relation to metabolic needs" by Cammilli, Alcidi and Papeschi which appeared in "Cardiac Pacing", pages 414-419 Amsterdam-Oxford: Excerpta Medica, 1977 which teaches sensing pH in the right atrium.
Another artificial cardiac pacemaker which increases pacing rate in accordance with an increase in respiration rate is proposed in the Krasner U.S. Pat. No. 3,593,718.
Pacers for sensing motion or mechanical activity are proposed in the Dahl U.S. Pat. No. 4,140,132 and the Anderson et al U.S. Pat. No. 4,428,378.
The Denniston III U.S. Pat. No. 3,815,611 discloses an apparatus which detects muscle contractions through impedance measurement. The device includes an elastomer body whose impedance changes when flexed. The elastomer body is positioned adjacent a muscle such as a heart muscle such that when the muscle contracts, the elastomer body is flexed to provide a change in impedance to a bias voltage supplied thereto. Such electrical signals can be used to control a pulse generator to generate a pulse when a specified period of time has elapsed since the latest heart activity was sensed by the elastomer body.
Heretofore it has been proposed in the Cohen U.S. Pat. No. 3,358,690 to sense pressure in the right atrium and to utilize the pressure sensed to control pacing of an electrode in the right ventricle.
Also, the Zacouto U.S. Pat. No. 3,857,399 discloses, in FIG. 19 thereof, a pressure sensor that measures either left ventricular pressure or intramyocardial pressure. One sensor is located in the myocardium or septum and the other sensor is located in the left ventricle. Apparently, the pacer coupled to these sensors responds to average base pressure over relatively long periods of time and the pacer system disclosed therein appears to be static and slowly responsive to exercise.
The Sjostrand et al. U.S. Pat. No. 3,650,277 discloses a system for reducing and controlling the blood pressure of a hypertensive patient by electrical stimulation of the carotid sinus nerves, one of the baroreceptor centers of the body. The system incorporates a pressure transducer which is connected to or applied to an artery of a patient and provides electrical signals substantially representing the instantaneous arterial blood pressure of a patient. Upon calculation of a mean arterial pressure, the system is utilized to provide a series of electrical pulses having a predetermined frequency, magnitude and amplitude through an afferent nerve, such as the carotid sinus nerve, to the heart to mimic pulses to the heart occurring naturally in patients having normal blood pressure. These pulses are provided during the first portion of each heart cycle to take over the function of controlling blood pressure that is usually provided by normally functioning baroreceptors in patients who are not hypertensive.
Another artificial cardiac pacemaker which is responsive to exercise by sensing venous blood temperature in the right ventricle of the heart is proposed in the Cook et al U.S. Pat. No. 4,43&,092.
As pointed out in the Alcidi U.S. Pat. No. 4,009,721, when an individual is engaging in muscular work or exerting a muscular effort, such as during exercise, the pH, the pO.sub.2 and the pCO.sub.2 of the human undergo a modification. More specifically, the pH and the pO.sub.2 decrease and the pCO.sub.2 increases. In view of this fact, Alcidi proposed monitoring the pH of the blood in the right atrium and to regulate the rate of stimulating pulses from a pacemaker in relation to the instantaneous variation of the pH of the blood.
Recently there has been proposed in the Bornzin U.S. Pat. No. 4,467,807 a rate adaptive demand pacemaker which alters the escape interval for demand pacing relative to a level of molecular oxygen in the oxygen-poor blood in the intracardiac or pulmonary artery. The molecular oxygen is sensed with an oxygen sensor, preferably of the type as disclosed in the Wirtzfeld et al U.S. Pat. Nos. 4,202,339 and 4,299,820. The Wirtzfeld et al patents teach measuring of oxygen saturation of blood using optical techniques. The transmissiveness of light through blood is used by Wirtzfeld et al to measure oxygen concentration. Bornzin teaches using such measurements for controlling the pacing of a heart.
It has also been proposed to provide a catheter which is capable of being inserted into a blood vessel and which has a gas permeable membrane and a pH sensor behind the membrane for measuring gases in the blood. Examples of such previously proposed catheters are disclosed in the following patents:
______________________________________ U.S. Pat. No. PATENTEE ______________________________________ 3,572,315 Cullen II 3,658,053 Fergusson et al 3,710,778 Cornelius ______________________________________
A percutaneous sensor for sensing pO.sub.2 permeating from the skin is disclosed in the Parker U.S. Pat. No. 4,197,853.
A field-effect transistor for detecting the concentration of oxygen in a substance is disclosed in the Janata U.S. Pat. No. 4,198,851.
The need for measuring the concentration of oxygen in biological fluids has led to the development of several oxygen sensing devices such as the Clark oxygen electrode. Such electrodes are used in polarographic measurements of oxygen and typically include a silver electrode (cathode) and a reference electrode (anode) made of a silver/silver chloride composition which are immersed in an alkaline solution containing diffused O.sub.2. An increasing voltage is applied to the cathode while the voltage of the reference electrode is maintained constant. A voltage-current curve generated in this manner will indicate the amount of diffusion of O.sub.2 which is directly related to pO.sub.2.
A catheter-mounted, oxygen polarographic electrode for use in testing of blood oxygen levels is disclosed in the Imredy U.S. Pat. No. 3,528,403.
Another cathode for polarographic measurements in a physiological medium such as human blood is disclosed in the Reichenberger U.S. Pat. No. 4,148,305.
Further, an electrode for measuring the concentration of oxygen gas, i.e. partial pressure of oxygen, in human blood is disclosed in the Uehara et al U.S. Pat. No. 4,442,841.
According to the teachings of the present invention, the partial pressure of oxygen pO.sub.2, in the blood is measured and such measurement is used to control the rate of pacing of an implanted artificial pacemaker system. Such a pacer or pacemaker system differs from the teachings of the Bornzin U.S. Pat. No. 4,467,807 by the measuring of partial pressure of oxygen as opposed to sensing oxygen saturation, by the mounting of a pO.sub.2 sensor in a pacing lead so that the partial pressure of oxygen in the blood in the heart is sensed and utilized for controlling the pacing of the heart.
Further differences in structure and operation of the apparatus and method of the present invention from the teachings of the various patents identified above will become apparent from the following description of the invention and the preferred embodiment thereof.